Absorption spectroscopy and fluorescence spectroscopy are commonly used to detect the presence of particular chemical species in a range of samples, especially dilute media such as gases. Absorption of narrow band laser light by a particular molecular transition may be detected by a reduction in detected intensity or amplitude especially in the ultraviolet or visible spectrum. Absorption of broad band light may be due to one or a plurality of molecular transitions and may also be detected by spectroscopic analysis of the light. Similarly, fluorescence arising from excited molecular states may also be detected.
However, such techniques typically rely on amplitude detection and so are susceptible to scintillation and laser noise.
“Resonant, heterodyne-laser-interferometer for state density measurements in atoms and ions”, Review of Scientific instruments 77, 093108 (2006), J. J. Moschella, et al., describes a resonant, two-wavelength heterodyne interferometer for measuring the population density of plasmas. Two separate lasers are used and an acousto-optical modulator splits the beams. However, this system is not particularly suitable for dilute species where the signal may be swamped by noise and other factors.
Therefore, there are required methods and apparatus for detecting dilute species that overcome these problems.